1. Field of the Invention
A protective alarm system for motor vehicle or the like is provided with a compact solid-state circuitry. Once actuated by a protective switch, the circuitry initiates the alarm after a predetermined delay and for a predetermined duration unless a definite succession of manual operations, involving the possession of a key, is carried out.
2. Description of the Prior Art
Burglar alarm systems are known comprising a first timing circuit which is operated by one or more protective switches and which in turn controls a second timing circuit, thereby to control the operation of an alarm circuit. Activation of the system is initiated by unauthorized tampering with a door, window or the like. A third key-operated timing circuit provides for activation of the protective circuit only after a predetermined time. See U.S. Pat. No. 3,659,266 to MEYERLE.
In such systems of the prior art, the alarm keeps operating for a predetermined time despite reclosing of the entrance. But if the reclosing occurs before the alarm is initiated (that is before the end of the first timing-circuit delay), the system resets itself in the initial conditions. Then, if a burglar or his accomplice opens and shuts the door with a period less than the time delay of the first timing circuit, the alarm is not triggered, and the burglar is able to operate at ease, as long as necessary for his evil-minded action.
Another drawback of a prior art burglar alarm system of that type is the considerable number of the components therein which are required by the structural or physical separation of the first and second timing circuits. SUMMARY OF THE INVENTION
An object of the present invention is a new circuit combining said first and second timing circuits, which results in an important reduction of the number of the components, thus reducing the cost and increasing the viability of the system.
The applicant has visualised for this purpose and has utilised a combined circuit which satisfies the two time-delay functions. It comprises a first capacitor shunted by a first resistance to introduce a time constant into the discharge, a second capacitor having one time constant with respect to the charge introduced by a second resistance and a second time constant with respect to the discharge introduced by the first resistance, and a diode shunting the second resistance. The first resistance and the second resistance are preferably adjustable.
Another object of the invention is to provide a key-operated timing circuit which is directly dependent on a key, for example the ignition key of the vehicle engine, and which is capable of being set by means of a two-position knob, so that a warning is triggered at the completion of the warning delay period when the delay-action triggering unit is set, whatever may be the position of the knob.
This circuit causes activation or deactivation of the protective system. It is included in the supply circuit of this protective system. It comprises an electronic gate which is formed of two transistors, a capacitor, the charge or discharge state of which controls the opening or closing of the electronic gate and means for charging or discharging the capacitor under the control of the two-position knob and a switch. The switch is preferably controlled by the ignition key of the vehicle.
For the purpose of charging the capacitor, it is necessary for the two-position knob to be closed for a period of time corresponding to the duration of the charging, and for the switch to be open (ignition key withdrawn); the delay-action control member is then set.
Once it is charged, the capacitor holds its charge for a considerable time, even if the switch is opened. Consequently, if a burglar gets into the vehicle and if he does not have the key for making contact before the warning-delay period expires, the alarm will be triggered on and will only be shut off at the end of the delay-action period, even if the burglar operates the two-position knob, which it will therefore not be necessary to conceal. The warning will not be stopped if the burglar cancels the means which has triggered the warning by reclosing the door for example.
In order to discharge the capacitor and thus to make the delay-action trigger device inoperative, it is necessary for the switch and the two-position knob to be both instantaneously closed.
The advantage of the two-position knob or button is that it permits the owner of the vehicle to cancel out, when necessary, the triggering of the warning signal without having to leave the ignition key in position.